1) Field of the Invention
This invention relates to an optical modulator suitable for use in the field of long distance optical communication systems.
2) Description of the Related Art
As the data transmission rate increases in recent years, optical modulators for modulating a data signal from an electric signal into an optical signal are developed energetically in the field of long distance communication systems for communication such as submarine optical communication.
One of such optical modulators as just described is, for example, such a single drive optical modulator 20 as shown in FIG. 8. Referring to FIG. 8, the single drive optical modulator 20 shown includes a substrate 21 on which a Mach-Zehnder optical waveguide 22 is formed, and an electrode 23 formed integrally on the substrate 21 and including a single signal electrode 23A and a grounding electrode 23B.
FIG. 9 is a sectional view taken along line A-Axe2x80x2 of the single electrode optical modulator 20 shown in FIG. 8. As seen in FIG. 9, the single electrode optical modulator 20 is configured such that the electrode 23 is formed on the substrate 21, which is made of, for example, lithium niobate (LiNbO3) and cut (Z-axis cut) in the Z-axis direction of the crystal orientation, together with the Mach-Zehnder optical waveguide 22.
The Mach-Zehnder optical waveguide 22 is formed by thermal diffusion of titanium (Ti) or a like substance on the substrate 21 and includes a Y branching waveguide 22A and two straight arm waveguides 22B-1 and 22B-2 on the light incoming side and a Y branching waveguide 22C on the light outgoing side. The electrode 23 includes the single signal electrode 23A and the grounding electrode 23B and converts, when a voltage signal (microwave) of, for example, NRZ (Non Return to Zero) data or the like is applied to the signal electrode 23A, the voltage signal into an NRZ optical signal.
As shown in FIG. 8, the single signal electrode 23A is formed so as to establish electric connection between two connector contacts on a one-side edge portion of the substrate 21 in its widthwise direction, and is formed such that part of it extends along and above the portion at which the straight arm waveguide 22B-1 is formed. Further, the grounding electrode 23B is formed such that it is disposed on the opposite sides of the single signal electrode in a spaced relationship by a predetermined distance thereby to form a coplanar line on the substrate 21.
When light from a light source not shown is introduced into the single electrode optical modulator 20 having the configuration described above with reference to FIGS. 8 and 9, while the light propagates in the Mach-Zehnder optical waveguide 22, it is modulated into an NRZ optical signal by the signal electrode 23A to which a voltage signal (microwave) of NRZ data or the like is applied. The modulated NRZ optical signal goes out of the single electrode optical modulator 20.
Where such a single electrode optical modulator 20 as described above is used to modulate a voltage signal into a data optical signal of a transmission rate particularly of 10 Gb/s or more, preferably of approximately 40 Gb/s, it is a significant subject for improvement of the transmission quality to suppress the loss of a microwave which advances through the electrode and suppress the deterioration of the extinction ratio.
It is an object of the present invention to provide an optical modulator which suppresses the loss of a microwave which advances through an electrode and loss of light which propagates in a waveguide and makes the losses in individual arm waveguides substantially equal to each other to suppress the deterioration of the extinction ratio to improve the transmission quality.
In order to attain the object described above, according to an aspect of the present invention, there is provided an optical modulator, comprising a substrate having an electro-optical effect and having formed thereon a ridge, first and second grooves which are positioned on the opposite sides of the ridge, and first and second banks which are positioned on the outer sides of the first and second grooves, respectively, a Mach-Zehnder optical waveguide formed on the substrate and including a first Y branching waveguide, first and second arm waveguides which are branched from the first Y branching waveguide and one of which is included in the ridge, and a second Y branching waveguide at which the first and second arm waveguides join together, an electrode formed on the substrate and including a signal electrode formed on the ridge and a grounding electrode formed on the first and second banks for controlling light propagating in the optical waveguide, and first and second recesses formed at symmetrical positions with respect to the ridge on the first and second banks, respectively.
With the optical modulator, since the first and second recesses are formed at the symmetrical positions with respect to the ridge 14a on the first and second banks, respectively, also the electric field distribution in the substrate by an electric signal provided to the signal electrode can be made symmetrical with respect to the ridge. Consequently, the optical modulator is advantageous in that the loss of a microwave which advances through the signal electrode can be suppressed.
Preferably, the substrate is made of LiNbO3, and more preferably, the substrate made of LiNbO3 is a Z-axis cut substrate.
The optical modulator may be configured such that the grounding electrode is provided on the first and second recesses and an air gap is formed in each of the first and second recesses or part of the grounding electrode is filled in the first and second recesses.
Preferably, the ridge and the first and second banks have top faces which are set in a substantially same level with one another, and more preferably, the first and second recesses have a depth set substantially equal to the depth of the first and second grooves.
Preferably, the signal electrode contacts with the ridge with a width smaller than the width of the ridge.
Preferably, a buffer layer is formed between the substrate and the electrode, and more preferably, the buffer layer is provided also in the first and second recesses.
Preferably, a silicon layer is placed on the substrate, and more preferably, the buffer layer is provided also in the first and second recesses.
The optical modulator is advantageous in that the absorption loss of light which propagates in the optical waveguide can be suppressed by the buffer layer and electric charge generated by a pyroelectric effect can be made uniform by the silicon layer to suppress the variation of the operating point by a temperature variation.
Further, since the buffer layer or the silicon layer is formed also in the first and second recesses 13-1, 13-2, the optical modulator is advantageous also in that adjustment of the characteristic impedance, which should be kept to a fixed value set in advance, and the speed matching between a microwave and light can be performed readily by setting of the thickness of the buffer layer or the silicon layer.
Preferably, one of the first and second arm waveguides is provided at a location of the other one of the first and second banks nearer to the ridge than a corresponding one of the first and second recesses.
With the optical modulator, since the one arm waveguide is provided nearer to the ridge than the other recess, the structure of the substrate portion at which the arm waveguide which is not included in the ridge is formed can be formed substantially same as the structure of the ridge. Therefore, the optical modulator is advantageous in that the losses of the arm waveguides can be made substantially equal to each other to suppress the deterioration of the extinction ratio.
According to another aspect of the present invention, there is provided an optical modulator, comprising a Z-axis cut substrate made of LiNbO3 and having formed thereon a ridge, first and second grooves which are positioned on the opposite sides of the ridge, and first and second banks which are positioned on the outer sides of the first and second grooves, respectively, a Mach-Zehnder optical waveguide formed on the substrate and including a first Y branching waveguide, first and second arm waveguides which are branched from the first Y branching waveguide and one of which is included in the ridge, and a second Y branching waveguide at which the first and second arm waveguides join together, an electrode formed on the substrate and including a signal electrode formed on the ridge and a grounding electrode formed on the first and second banks for controlling light propagating in the optical waveguide, a buffer layer formed between the substrate and the electrode, a silicon layer placed on the substrate, and first and second recesses formed at symmetrical positions with respect to the ridge on the first and second banks, respectively.
With the optical modulator, since the first and second recesses are formed at the symmetrical positions with respect to the ridge 14a on the first and second banks, respectively, also the electric field distribution in the substrate by an electric signal provided to the signal electrode can be made symmetrical with respect to the ridge. Consequently, the optical modulator is advantageous in that the loss of a microwave which advances through the signal electrode can be suppressed.
Preferably, the buffer layer or the silicon layer is provided also in the first and second recesses. In this instance, the optical modulator is advantageous also in that adjustment of the characteristic impedance, which should be kept to a fixed value set in advance, and the velocity match between a microwave and light can be performed readily by setting of the thickness of the buffer layer or the silicon layer.
Preferably, one of the first and second arm waveguides is provided at a location of the other one of the first and second banks nearer to the ridge than a corresponding one of the first and second recesses. In this instance, the structure of the substrate portion at which the arm waveguide which is not included in the ridge is formed can be formed substantially same as the structure of the ridge. Therefore, the optical modulator is advantageous in that the losses of the arm waveguides can be made substantially equal to each other to suppress the deterioration of the extinction ratio.
The above and other objects, features and advantages of the present invention will become apparent from the following description and the appended claims, taken in conjunction with the accompanying drawings.